Image transform apparatus and image transform program

ABSTRACT

It is an object of the present invention to provide an image transform apparatus and an image transform program which can reduce a computation load for transforming a luminance image to a brightness image. The present invention attains the object by performing wavelet resolution of the luminance image first to generate J pieces (J is an integer equal to two or more) of sub band images, transforming a luminance value of each pixel of the sub band images to a brightness impression value based on a predetermined relation between luminance and brightness impression, and then performing wavelet synthesis of K pieces (K is an integer equal to two or more; K≦J) of sub band images having been subjected to the transformation of the luminance values to the brightness impression values, to generate a brightness image.

CROSS REFERENCE TO RELATED APPLICATION

This application is a US nationalization of International ApplicationPCT/JP20006/304242, filed Mar. 6, 2006 which claims the benefit ofpriority from Japanese Patent Application No. 2005-165191, filed Jun. 6,2005.

TECHNICAL FIELD

The present invention relates to an image transform apparatus and animage transform program for designing a luminance condition.

BACKGROUND ART

Conventionally, a lighting condition (that is, luminance distribution ina scene) which is produced depending on the layout and so on of lightingequipments has been simulated. An image with luminance distributionpredicted from this lighting simulation is utilized for designing theluminance condition as a reference image.

However, how a person perceives brightness (referred to as “brightnessimpression” herein) does not directly correspond to values of luminance.For example, when in a luminance image, a peripheral area with lowerluminance than an object area is compared with a peripheral area withhigher luminance than an object area, the person perceives the formerobject area as brighter than the latter, even when both the object areasare equal in luminance. Note that the brightness impression is sometimescalled brightness perception.

Therefore, predicting accurate luminance distribution by the abovelighting simulation and referring to an image with the obtainedluminance distribution are not sufficient to study a luminance conditionat the time of designing it. This is because brightness impression of anobject area changes depending on luminance of a peripheral area evenwith no change in the luminance of the object area.

In order to study the luminance condition sufficiently, in recent yearsthere has been a demand for quantitative prediction of brightnessimpression of an object area of a luminance image. The present inventorshave proposed a technique to quantitatively predict brightnessimpression of an object area with high accuracy even in a case whereluminance distribution is complicated as in an actual scene (see, forexample, a patent document 1). Further, applying this techniqueextensively makes it possible to transform a luminance image to abrightness image.

Patent document 1 Japanese Unexamined Patent Application Publication No.2004-61150

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

However, the aforesaid technique has a problem of an enormouscomputation load for transforming the luminance image to the brightnessimage, and it needs a long calculation time to obtain the brightnessimage.

It is an object of the present invention to provide an image transformapparatus and an image transform program which can reduce a computationload for transforming a luminance image to a brightness image.

Means for Solving the Problems

An image transform apparatus of the present invention includes aresolution device which performs wavelet resolution of a luminance imageto generate J pieces (J is an integer equal to two or more) of sub bandimages; a transform device which transforms a luminance value of eachpixel of the sub band images to a brightness impression value based on apredetermined relation between luminance and brightness impression; anda synthesis device which performs wavelet synthesis of K pieces (K is aninteger equal to two or more; K≦J) of sub band images which have beensubjected to the transformation of the luminance values to thebrightness impression values by the transform device, to generate abrightness image.

Another image transform apparatus of the present invention includes aresolution device which performs wavelet resolution of a luminance imageN times (N is a natural number) to generate (3N+1) pieces of sub bandimages; a transform device which transforms a luminance value of eachpixel of the sub band images to a brightness impression value based on apredetermined relation between luminance and brightness impression; anda synthesis device which performs wavelet synthesis of (3N+1) pieces ofsub band images N times to generate a brightness image, the (3N+1)pieces of sub band images having been subjected to the transformation ofthe luminance values to the brightness impression values by thetransform device.

Preferably, the resolution device performs the wavelet resolution usingan orthogonal wavelet, and the synthesis device performs the waveletsynthesis using the orthogonal wavelet.

Preferably, the orthogonal wavelet is an approximately symmetricalfunction.

Preferably, the image transform apparatus includes a second resolutiondevice which performs wavelet resolution of the brightness imagegenerated by the synthesis device to generate J′ pieces (J′ is aninteger equal to two or more) of sub band images; a second transformdevice which transforms, based on the relation, the brightnessimpression value of each of the pixels of the sub band images generatedby the second resolution device to the luminance value; and a secondsynthesis device which performs wavelet synthesis of K′ pieces (K′ is aninteger equal to two or more; K′≦J′) of sub band images which have beensubjected to the transformation of the brightness impression values tothe luminance values by the second transform device, to generate aluminance image.

Preferably, the image transform apparatus includes a second resolutiondevice which performs wavelet resolution of the brightness imagegenerated by the synthesis device N′ times (N′ is a natural number) togenerate (3N′+1) pieces of sub band images; a second transform devicewhich transforms, based on the relation, the brightness impression valueof each of the pixels of the sub band images generated by the secondresolution device to the luminance value; and a second synthesis devicewhich performs wavelet synthesis of (3N′+1) pieces of sub band imageswhich have been subjected to the transformation of the brightnessimpression values to the luminance values by the second transformdevice, to generate a luminance image.

An image transform program of the present invention includes aresolution step of performing wavelet resolution of a luminance image togenerate J pieces (J is an integer equal to two or more) of sub bandimages; a transform step of transforming a luminance value of each pixelof the sub band images to a brightness impression value based on apredetermined relation between luminance and brightness impression; anda synthesis step of performing wavelet synthesis of K pieces (K is aninteger equal to two or more; K≦J) of sub band images which have beensubjected to the transformation of the luminance values to thebrightness impression values by the transform step, to generate abrightness image.

Another image transform program of the present invention causes acomputer to execute: a resolution step of performing wavelet resolutionof a luminance image N times (N is a natural number) to generate (3N+1)pieces of sub band images; a transform step of transforming a luminancevalue of each pixel of the sub band images to a brightness impressionvalue based on a predetermined relation between luminance and brightnessimpression; and a synthesis step of performing wavelet synthesis of(3N+1) pieces of sub band images N times to generate a brightness image,the (3N+1) pieces of sub band images having been subjected to thetransformation of the luminance values to the brightness impressionvalues by the transform step.

ADVANTAGEOUS EFFECT OF THE INVENTION

According to the image transform apparatus and the image transformprogram of the present invention, it is possible to reduce a computationload for transforming a luminance image to a brightness image.

BRIEF DESCRIPTION OF THE DRAWINGS

The nature, principle, and utility of the invention will become moreapparent from the following detailed description when read inconjunction with the accompanying drawings in which like parts aredesignated by identical reference numbers, in which:

FIG. 1 is a block diagram showing a schematic structure of an imagetransform apparatus 10 of this embodiment;

FIG. 2 is a flowchart describing the image transformation step;

FIG. 3 are views explaining wavelet resolution;

FIG. 4 is an explanatory chart of sub band images generated by thewavelet resolution;

FIG. 5 shows an example of coefficients representing the relationbetween luminance and brightness impression;

FIG. 6 is an explanatory chart of sub band images which have undergonecoefficient processing;

FIG. 7 is an explanatory chart of a rating scale of brightnessimpression;

FIG. 8 are charts describing patterns used in accuracy evaluation;

FIG. 9 are schematic views showing the structure of an experimentalapparatus used to measure brightness impression;

FIG. 10 is a chart showing accuracy of prediction of brightnessimpression when the patterns in FIG. 8 are used;

FIG. 11 is an explanatory chart of changes in brightness impression(measured values) depending on the size of an object area; and

FIG. 12 is an explanatory chart of changes in brightness impression(measured values) depending on the size of the object area.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, an embodiment of the present invention will be described indetail.

An image transform apparatus 10 (FIG. 1) of this embodiment is acomputer in which an image transform program is installed, and itincludes a memory 10A which stores input images as objects to beprocessed (for example, luminance images) and output images (forexample, brightness images) as processing results; and a computationunit 10B which executes computation processing of image transformationaccording to the steps of the flowchart shown in FIG. 2. To install theimage transform program in the computer, a recording medium (CD-ROM orthe like) on which the image transform program is recorded is used.Alternatively, a carrier wave (including the image transform program)downloadable via the Internet may be used.

The image transform apparatus 10 captures a luminance image (FIG. 3( a))into the memory 10A at Step S1 in FIG. 2. The luminance image is adigital image relating to luminance distribution of a scene predicted byarbitrary lighting simulation and represents scene dependency of aluminance real value. The lighting simulation is simulation of aluminance condition which is produced depending on, for example, thelayout and the like of lighting equipments in a room. The luminanceimage can be captured not only through the lighting simulation but alsoby using a CCD camera.

Next, after calculating a logarithm of each pixel value of the luminanceimage, the computation unit 10B of the image transform apparatus 10proceeds to processing at Step S2. At Step S2, wavelet resolution of theluminance image (logarithmic image) is performed by using an orthogonalwavelet (for example, symlet6), whereby four sub band images LL(−1),HL(−1), LH(−1), HH(−1) at −1 level shown in FIG. 3( b) are generated.The sub band image LL(−1) is a low-frequency component extracted from aluminance change in the luminance image, and can be considered as anapproximate image of the luminance image. The other sub band imagesHL(−1), LH(−1), HH(−1) are a high-frequency component in a verticaldirection, a high-frequency component in a horizontal direction, and ahigh-frequency component in an oblique direction, respectively,extracted from the luminance change in the luminance image. The numberof pixels in each of the sub band images LL(−1), HL(−1), LH(−1), HH(−1)is ¼ of the number of pixels of the luminance image.

Next (Step S3), the image transform apparatus 10 determines whether ornot the level of the aforesaid wavelet resolution has reached the lowestlevel, and if the lowest level has not been reached, it returns to theprocessing at Step S2 to repeat the wavelet resolution while loweringthe level in decrements of one level. In this embodiment, the lowestlevel is, for example, −11 level. In this case, the processing at StepS2 (wavelet resolution) is repeated 11 times.

The second wavelet resolution is wavelet resolution from the −1 level to−2 level and is performed on the sub band image LL(−1) which is thelow-frequency component at the −1 level. As a result, four sub bandimages LL(−2), HL(−2), LH(−2), HH(−2) at the −2 level shown in FIG. 3(c) are generated. The sub band image LL(−2) is a low-frequency componentextracted from a luminance change in the sub band image LL(−1) and canbe considered as an approximate image of the luminance image similarlyto the sub band image LL(−1). The other sub band images HL(−2), LH(−2),HH(−2) are a high-frequency component in the vertical direction, ahigh-frequency component in the horizontal direction, and ahigh-frequency component in the oblique direction, respectively,extracted from the luminance change in the sub band image LL(−1).

As compared with the first wavelet resolution (FIG. 3( a)→(b)), thesecond wavelet resolution (FIG. 3( b)→(c)) can extract a lower-frequency(rougher) luminance change. Similarly, in the wavelet resolutions at andafter the third time, four sub band images LL(−3), HL(−3), LH(−3),HH(−3) at −3 level are generated from the sub band image LL(−2) which isa low-frequency component at the −2 level (FIG. 3( d)), and each timethe level is lowered, a rougher luminance change is extracted. Then,when four sub band images LL(−11), HL(−11), LH(−11), HH(−11) at the −11level are generated, the image transform apparatus 10 proceeds to thenext Step S4.

At this instant, the wavelet resolution has been repeated eleven times,and 34 sub band images HL(−1), LH(−1), HH(−1), HL(−2), LH(−2), HH(−2), .. . , LL(−11), HL(−11), LH(−11) HH(−11) shown in FIG. 4 have beengenerated to be stored in the memory 10A of the image transformapparatus 10.

Next (Step S4), the computation unit 10B of the image transformapparatus 10 performs the following coefficient processing based on apredetermined relation between luminance and brightness impression (forexample, coefficients α(−1), α(−2), . . . , α(−11), β(−11) for therespective levels shown in FIG. 5) to transform a value of luminance ofeach pixel of the sub band images HL(−1), LH(−1), HH(−1), HL(−2),LH(−2), HH(−2), . . . LL(−11), HL(−11), LH(−11), HH(−11) to a value ofbrightness impression.

Concretely, according to the following equation (1) using thecoefficient α(−11) for the −11 level, pixel values (values of luminance)of the sub band image LL(−11) which is a low-frequency component at the−11 level are transformed to the values of brightness impression (pixelvalues of a sub band image LL′(−11)).pixel value of LL′(−11)=β(−11)×(pixel value of LL(−11))+4.653435  (1)

The sub band image LL′(−11) corresponds to a low-frequency component atthe −11 level of a brightness image which is to be finally obtained.

Further, according to the following equations (2)˜(4) using thecoefficient α for −N level (N=1˜11), pixel values (values of luminance)of the sub band images HL(−N), LH(−N), HH(−N) which are high-frequencycomponents at the −N level are transformed to values of brightnessimpression (pixel values of sub band images HL′(−N), LH′(−N), HH′(−N)).The sub band images HL′(−N), LH′(−N), HH′(−N) correspond tohigh-frequency components at the −N level of the brightness image whichis to be finally obtained.pixel value of HL′(−N)=α(−N)×(pixel value of HL(−N))  (2)Pixel value of LH′(−N)=α(−N)×(pixel value of LH(−N))  (3)pixel value of HH′(−N)=α(−N)×(pixel value of HH(−N))  (4)

The coefficient processing (Step 54) based on the equations (1)˜(4) asdescribed above is processing to add an effect which is given to thebrightness impression by the luminance change with various frequenciesextracted from the original luminance image (that is, the coefficientsα(−1), α(−2), . . . , α(−11), β(−11)).

As a result, sub band images HL′(−1), LH′(−1), HH′(−1), . . . , LL′(−1),HL′(−11), LH′(−11), HH′(−11) shown in FIG. 6 are stored in the memory10A of the image transform apparatus 10. Note that the pixel values(values of the brightness impression) of the sub band image LL′(−11)correspond to brightness impression (Bu) when luminance is uniform. Thepixel values of the sub band images HL′(−N), LH′(−N), HH′(−N) (values ofthe brightness impression) correspond to brightness impression (Bc) bythe contrast effect of luminance. How the coefficients α(−1), α(−2), . .. , α(−11), β(−11) are decided will be described finally.

Next (Step S5), the computation unit 10B of the image transformapparatus 10 performs wavelet synthesis of the four sub band imagesLL′(−11), HL′(−11), LH′(−11), HH′(−11) at the −11 level shown in FIG. 6by using the same orthogonal wavelet (for example, symlet6) as that usedat Step S2. A sub band image LL′(−10) which is a low-frequency componentat one level higher (−10) can be generated by this wavelet synthesis.

Next (Step S6), it is determined whether or not the level of theaforesaid wavelet synthesis has reached the level of the original image(here, the luminance image in FIG. 3( a)), and if the level of theoriginal image has not been reached, the image transform apparatus 10returns to the processing at Step S5 to repeat the wavelet synthesiswhile raising the level in increments of one level. In this embodiment,since the lowest level is the −11 level, the processing at Step S5(wavelet synthesis) is repeated eleven times.

The second wavelet synthesis is wavelet synthesis from the −10 level to−9 level and is performed by using the sub band image LL′(−10) generatedby the first wavelet synthesis and the three sub band images HL′(−10),LH′(−10), HH′(−10) at the −10 level shown in FIG. 6. As a result, a subband image LL′(−9) which is a low-frequency component at one levelhigher (the −9 level) can be generated.

The wavelet syntheses at and after the third time are also performed inthe same manner, and when a sub band image (that is, a brightness image)which is a low-frequency component of the level of the original image (0level) is generated based on a sub band image LL′(−1) at the −1 levelgenerated by the tenth wavelet synthesis and the three sub band imagesHL′(−1), LH′(−1), HH′(−1) at the −1 level shown in FIG. 6, the imagetransform apparatus 10 finishes the computation processing of the imagetransform in FIG. 2.

Pixel values (values of brightness impression) of the brightness imagethus generated as a result of the transform of the luminance image (FIG.3( a)) are obtained as numerical values (1˜13) of a rating scale shownin FIG. 7. Therefore, by knowing adjectives (very dark˜very bright)corresponding to the pixel values (1˜13) of the brightness image, it ispossible to find “the brightness impression” adapted to humanperception. In the rating scale in FIG. 7, the numerical values (1˜13)are allotted to the adjectives (very dark˜very bright) of the brightnessimpression generally used in a field of the design of a luminancecondition, for convenience sake.

(Accuracy Evaluation)

In order to evaluate accuracy of the transform by the image transformapparatus 10 of this embodiment, pixel values (predicted values) of thebrightness image resulting from the transform from the luminance image(FIG. 3( a)) are compared with measured values obtained from subjects ina later-described experiment.

A pattern prepared as a luminance image for evaluation has two areas (anobject area, a peripheral area) different in luminance as shown in FIG.8( a) and the size of the object area is variable (visual angle 0.1°,0.5°, 1°, 2°, 5°, 10°, 15°, 20°, 30°, 60°, 180°). Further, thecombination of luminance [cd/m²] of the object area and a luminanceratio C (=object luminance/periphery luminance) between the object areaand the peripheral area is changed as shown in FIG. 8( b). Then, thesepatterns are used for the evaluation.

Predicted values of the brightness impression of the object areas of therespective patterns are found by the image transform apparatus 10according to the above-described steps of the flowchart shown in FIG. 2.

An experimental apparatus shown in FIGS. 9( a), (b) is used to measurethe brightness impression of the object area of each of the patterns. Asshown in FIG. 9( a), the experimental apparatus includes: an opaquewhite panel 11 covering a 180° visual angle; a cylinder 12 of blackpaper attached to the panel 11 from a center portion to an outer side ofthe panel 11; a neutral white fluorescent lamp 13 attached to a tipportion of the cylinder 12; a film 15 attached to the other end (panel11 side) of the cylinder 12; and a large number of fluorescent lamps 14provided along the whole outer side of the panel 11.

The brightness impression was measured by using this experimentalapparatus in a room light-shielded by a blackout curtain while theluminance of the peripheral area is adjusted by light intensity of thefluorescent lights 14, the luminance of the object area is adjusted bylight intensity of the fluorescent lamp 13, and the size of the objectarea was adjusted. Further, the luminance of the peripheral area nearthe object area was adjusted by changing transmittance of the film 15.The subjects were totally nine men and women in their 20's whosetwenty-twenty vision including corrected vision by eyeglasses or thelike was 1.0 or more. The actual measurement was repeated twice or threetimes (/one subject) for each of the patterns so as to obtain stablerating. The patterns of various kinds were presented at random to thesubjects.

Further, in the measurement of the brightness impression by using thisexperimental apparatus, each of the subjects selects one from thenumerical values (1˜13) of the rating scale in FIG. 7 as brightnessimpression of the object area of each of the presented patternsaccording to how he/she perceives. The numerical values of the ratingscale selected by the respective subjects are averaged for each of thepatterns, and the obtained average values are defined as the “measuredvalues”.

The comparison between the predicted values and the measured values ofthe brightness impression for the respective patterns obtained asdescribed above are shown in FIG. 10. In FIG. 10, the horizontal axisshows the measured value and the vertical axis shows the predictedvalue. As is seen from FIG. 10, points (♦) each representing thecombination of the measured value and the predicted value of each of thepatterns mostly concentrate on a 45° line. Errors are within about ±1 interms of the rating scale. Further, the R2 value shows full explanatorypower of 92% (R=0.959582).

From the above result, it can be said that the accuracy of thecomputation processing (FIG. 2) of the image transform in the imagetransform apparatus 10 of this embodiment is sufficiently high. That is,according to the image transform apparatus 10 of this embodiment, it ispossible to quantitatively predict brightness impression of an objectarea in a luminance image with high accuracy even in a case whereluminance distribution is complicated.

Further, according to the image transform apparatus 10 of thisembodiment, since the luminance image is transformed to the brightnessimage by the combination of the wavelet resolution at Step S2 and thewavelet synthesis at Step S5 in FIG. 2 (that is, by applying stationarywavelet transform), a computation load of this transform can be reduced.Therefore, it is possible to obtain the brightness image from theluminance image in a far shorter time and at higher speed than in aconventional art.

Further, according to the image transform apparatus 10 of thisembodiment, since the orthogonal wavelet is used for the waveletresolution and the wavelet synthesis (Steps S2, S5 in FIG. 2), it ispossible to greatly reduce an error in the computation processing of theimage transformation. Therefore, by performing inverse transformationfrom the brightness image to the luminance image by using the sameorthogonal wavelet after the above-described transform from theluminance image to the brightness image, it is possible to restore theoriginal luminance image. That is, according to the image transformapparatus 10, high-speed bi-directional transform to/from the luminanceimage from/to the brightness image is enabled.

Here, the inverse transformation from the brightness image to theluminance image will be described. Processing of this inversetransformation is processing in which, in the description of the abovetransform processing (FIG. 2 to FIG. 6), the brightness image issubstituted for the luminance image and the luminance image issubstituted for the brightness image, and the following equations(5)˜(8) are used as bases of the coefficient processing at Step S4 inFIG. 2. LL′(−N), HL′(−N), LH′(−N), HH′(−N) are sub band images at the −Nlevel of the brightness image. LL(−N), HL(−N), LH(−N), HH(−N) are subband images at the −N level of the luminance image.pixel value of LL(−11)={(pixel value of LL′(−11))−4.653435}÷β(−11)  (5)pixel value of HL(−N)=(pixel value of HL′(−N))÷α(−N)  (6)pixel value of LH(−N)=(pixel value of LH′(−N))÷α(−N)  (7)pixel value of HH(−N)=(pixel value of HH′(−N))÷α(−N)  (8)

This coefficient processing based on the equations (5)˜(8) is alsoprocessing to add the effect given to the brightness impression by thebrightness change with various frequencies (that is, α(−1), α(−2), . . ., α(−11), β(−11)).

The sub band images LL′(−1), . . . generated by the wavelet resolutionare low-frequency components or high-frequency components extracted fromthe brightness change in the brightness image. The pixel values of thesub band image LL(−11) correspond to uniform luminance. The pixel values(values of luminance) of the sub band images HL(−N), LH(−N), HH(−N)correspond to the contrast effect of luminance.

The inverse transformation processing as described above may be appliedto a brightness image which is generated in advance by transforming theluminance image, or may be applied to a brightness image newly generatedby some method. In the former case, it is preferable to use the sameorthogonal wavelet as that used when the luminance image is changed tothe brightness image. Further, at the time of the inverse transformationfrom the brightness image to the luminance image, each pixel of theluminance image at an instant when the wavelet synthesis is finishedpresents a logarithmic value. Therefore, it is preferable to performcalculation for returning each of the pixel values to a real value anddefine the resultant image as the final luminance image.

In the image transform apparatus 10 of this embodiment, it is possibleto generate the brightness image by transforming the pixel values(values of luminance) of the sub band images to the values of thebrightness impression according to the equations (1)˜(4) when thewavelet resolution of the luminance image is performed and by performingthe wavelet synthesis of the sub band images resulting from thetransform. Further, it is possible to generate the luminance image bytransforming the pixel values (values of the brightness impression) ofthe sub band images according to the equations (5)˜(8) when the waveletresolution of the brightness image is performed and by performing thewavelet synthesis of the sub band images resulting from the transform.

Further, in the image transform apparatus 10 of this embodiment, sincethe bi-directional transform to/from the luminance image from/to thebrightness image as described above can be performed at high speed, itis possible to efficiently realize lighting design, lighting control,and the like as follows.

Conventional lighting design is lighting design using illuminance andcan ensure easy view of characters and so on written on paper, but itcan hardly cope with the design of lighting for producing someatmosphere in a room, wall lighting in an open ceiling space,illumination for lighting up, and the like (brightness distributionproduced by light). On the other hand, according to this embodiment, itis possible to obtain a physical quantity (luminance) of illuminationrealizing the above design while directly adjusting the pixel values(brightness impression) of the brightness image. Therefore, a designercan design output, light intensity distribution, positions, and so on oflighting equipments as desired. In this case, it is conceivable toinstall the computation processing of the image transformation(bi-directional transform) in CG generating software.

Further, in recent years, introducing natural light into a room has beenconsidered in various fields in view of energy saving. At this time,light intensity of artificial lighting needs to be adjusted according tothe introduction of the natural light. Conventionally, since a degree ofthe introduction of the natural light has been measured by anilluminometer, it has been difficult to appropriately adjust the lightintensity of artificial lighting. However, according to this embodiment,it is possible to capture a luminance image in real time by using a CCDcamera and transform the luminance image to a brightness image (imagedirectly expressing how a person perceives), and therefore, adjustingthe artificial lighting according to the pixel values (values of thebrightness impression) of the brightness image can realize an optimumluminance condition produced by the combination of the natural light andthe artificial lighting. In this case, it is conceivable to install thecomputation processing of the image transformation (bi-directionaltransformation) in a control system for the utilization of the naturallight.

Further, the same control system is also usable not only for theabove-described adjustment of the artificial lighting but also for theadjustment of the inclination of a window blind, the output adjustmentof monitor devices of various apparatuses, the output adjustment of a PCprojector, and the like.

Further, according to the image transform apparatus 10 of thisembodiment, since the approximately symmetrical function (for example,symlet6) is used as the orthogonal wavelet at the time of the waveletresolution of the luminance image and the brightness image and at thetime of the wavelet synthesis (Steps S2, S5 in FIG. 2), the transformappropriate for a characteristic of the brightness impression of humaneyes (characteristic that a contrast effect of the brightness impressionis symmetrical with respect to the center of a field of view and doesnot have directivity) is enabled.

(Supplement)

Finally, how the coefficients α(−1), α(−2), . . . , α(−11), β(−11) inFIG. 5 are decided will be described.

Patterns prepared as luminance images for this purpose are the same asthose in FIGS. 8( a), (b) described above.

FIG. 11 and FIG. 12 show measured values of the brightness impression ofthe respective patterns, for example, in a case where a luminance ratiois 100 and those in a case where a luminance ratio is 0.3. In FIG. 11and FIG. 12, the horizontal axis is a logarithmic axis showing the size(deg′) of an object area and the vertical axis shows the measured valueof the brightness impression. In FIG. 11 and FIG. 12, “x”, “▴”, “▪”,“●”, “♦” correspond to measured values when the luminance of the objectarea is 3000, 300, 30, 3, 0.3 [cd/m²] respectively.

As is seen from FIG. 11, in the case where the luminance ratio>1 (theobject area has higher luminance), the brightness impression presents adecreasing tendency as the object area increases in size. On the otherhand, as is seen from FIG. 12, in the case where the luminance ratio<1(the object area has lower luminance), the reverse tendency is observedand the brightness impression increases as the object area increases insize.

Further, that the size of the object area is “∝” corresponds to that thevisual angle of the object area of each of the patterns is 180°. Thatis, it means a state where object luminance distributes uniformly allover a 180° field of view. Therefore, the measured values when the sizeof the object area is “∝” correspond to the brightness impression (Bu)when luminance is uniform.

Further, brightness impression (Bc) by a contrast effect of luminancecan be thought to correspond to a difference between the measured valuewhen the size of the object area is “∝” (see Bu in FIG. 11 and FIG. 12)and the measured value when the size is limited. As shown in FIG. 11, inthe case where the luminance ratio>1 (the object area has higherluminance), the brightness impression Bc by the contrast effect ofluminance has a “plus” value, and as shown in FIG. 12, in the case wherethe luminance ratio<1 (the object area has lower brightness), thebrightness impression Bc by the contrast effect of luminance has a“minus” value.

Therefore, by using the measured values of the brightness impression ofthe respective patterns (FIG. 11, FIG. 12) to perform multi regressionanalysis of the measured values on assumption that symlet6 is used asthe orthogonal wavelet and also on assumption that resolution (visualangle per one pixel) of the luminance image is about 0.1°, it ispossible to decide values of the coefficients α(−1), α(−2), . . . ,α(−11), β(−11) in FIG. 5. It is also possible to decide the value(4.653435) of the constant terms in the equations (1), (5).

Modified Examples

In the above-described embodiment, symlet6 is used as the orthogonalwavelet, for instance, but the present invention is not limited to this.It is also possible to perform the same calculation by using any otherapproximately symmetrical function (for example, symlet4, 8 or thelike). However, in a case where a function other than symlet6 is used,values different from the values in FIG. 5 and the equations (1), (5)need to be newly found as coefficients representing the relation betweenluminance and brightness impression.

Further, in the above-described embodiment, the same orthogonal waveletis used at the time of the transform from the luminance image to thebrightness image and at the time of the inverse transformation from thebrightness image to the luminance image, but different orthogonalwavelets may be used. In this case, since appropriate coefficients(relation between luminance and brightness impression) exist for each ofthe orthogonal wavelets, it is necessary to find the appropriatecoefficients by the above-described method and use them for thecomputation processing of the image transform.

The above-described embodiment uses the orthogonal wavelet, but thepresent invention is also applicable to a case where a nonorthogonalwavelet is used. In this case, since functions are not independent,approximation calculation is required for the wavelet synthesis.

However, with use of a completely restorable wavelet obtained byappropriately combining a wavelet for the resolution and a wavelet forthe synthesis (that is, a biorthogonal wavelet), the approximationcalculation is not required even when the wavelets of theresolution/synthesis are nonorthogonal. Therefore, with the use of thebiorthogonal wavelet, it is also possible to greatly reduce an error inthe computation processing of the image transformation and to performthe bi-directional transform to/from the luminance image from/to thebrightness image at high speed, similarly to the above-described casewhere the orthogonal wavelet is used. Thus, the present invention iseffective and can provide the same effects not only in the case whereeach of the wavelets of the resolution/synthesis is orthogonal but alsoin the case where the combination of the both wavelets is orthogonal(biorthogonal).

In the above-described embodiment, the wavelet resolution is performedup to the −11 level, but the present invention is not limited thereto.The lowest level may be set according to required accuracy of the imagetransformation. Further, in a case where the size of an original image(for example, a luminance image) is small and a sub band image has onepixel at a stage before the level reaches the −11 level, the waveletresolution may be finished at this instant. In any case, in a case wherethe lowest level at which the wavelet resolution is finished isdifferent from the −11 level, the following equations (9), (10) arecalculated instead of the calculation of the equations (1), (5), foreach pixel of the sub band image (LL) which is a low-frequency componentat the lowest level. In the equations (9), (10), −M level is the lowestlevel.pixel value of LL′(−M)=β(−M)×(pixel value of LL(−M))+4.653435  (9)pixel value of LL(−M)={(pixel value of LL′(−M))−4.653435}÷β(−M)  (10)

The wavelet resolution may be continued even after the sub band imagehas one pixel to be finished at an instant when the level reaches the−11 level. In the coefficient processing in this case, the aforesaidequations (1)˜(8) are used.

Further, the values of the coefficients α(−1), α(−2), . . . , α(−11),β(−11) representing the relation between luminance and brightnessimpression and the values of the constant terms in the equations (1),(5) are preferably found for each kind of the orthogonal wavelets, andin addition, in a case where resolution of an original image (forexample, a luminance image) is changed, they are preferably found foreach resolution.

Further, the above embodiment has described the example where the imagetransform apparatus 10 is a computer in which the image transformprogram is installed, but the present invention is not limited to this.The image transform apparatus 10 may be structured as a chip bydedicated hardware (LSI). By structuring the image transform apparatus10 as a chip, lighting control and so on can be made real-time control.

In the above-described embodiment, at the time of the transform from theluminance image to the brightness image, the wavelet resolution isperformed after the logarithmic values of the pixel values (real values)of the luminance image are calculated, and at the time of the inversetransformation from the brightness image to the luminance image, thefinal luminance image is generated by calculating the real values of thepixel values (logarithmic values) of the luminance image immediatelyafter the wavelet synthesis, but the present invention is not limited tothis. The calculation of the logarithms as described above iscalculation in which nonlinearity of a visual system is taken intoconsideration, and the same effect can be also obtained when a powerfunction such as, for example, ⅓ power is used other than thelogarithms. The power function may be set according to the expression ofa uniform perception space.

In the above-described embodiment, the results of a psychological ratingexperiment based on the absolute scale are used for the brightnessperception, but threshold values (borderline values above and belowwhich a difference is recognizable and not recognizable) or the like maybe used for the scale.

In the above-described embodiment, the resolution of an image is set toabout 0.1°, but the present invention is not limited to this. Theresolution may be set to any value (for example, higher resolution)other than 0.1°. However, in a case where the setting of the resolutionis changed, it is necessary to find, for each resolution, thecorrespondence relation (coefficients) with the perception scale.

In the above-described embodiment, the wavelet resolution is repeated Ntimes (N is a natural number), the (3N+1) pieces of sub band images aregenerated by N times of the wavelet resolution, thereafter, all of thesesub band images are subjected to the coefficient processing, and thewavelet synthesis (N times) of all the (3N+1) pieces of sub band imageshaving undergone the processing is performed, but the present inventionis not limited to this. Sub band images in an arbitrary plural numbersmaller than (3N+1) out of the sub band images having undergone thecoefficient processing may be used for the wavelet synthesis. However,using all the sub band images having undergone the coefficientprocessing for the wavelet synthesis enables more accurate transform,and enables the bi-directional transform to/from the luminance imagefrom/to the brightness image.

In the above-described embodiment, four sub band images are generated byone wavelet resolution, but the present invention is not limited tothis. The present invention is applicable to any case where the numberof sub band images generated by one wavelet resolution is two or more.Similarly, a sub band image at one level higher is generated from foursub band images by one wavelet synthesis, but a sub band image at ahigher level may be generated from two sub band images or more.

In the above-described embodiment, the numbers of times the waveletresolution is performed at the time of the bi-directional transformto/from the luminance image from/to the brightness image are set equalto each other (for example, 11 times), but the present invention is notlimited to this. The number of times the wavelet resolution is performedat the time of the transform from the luminance image to the brightnessimage and the number of times the wavelet resolution is performed at thetime of the inverse transformation from the brightness image to theluminance image may be set different.

The invention is not limited to the above embodiments and variousmodifications may be made without departing from the spirit and scope ofthe invention. Any improvement may be made in part or all of thecomponents.

1. An image transform apparatus comprising: a resolution device whichperforms wavelet resolution of a luminance image to generate J pieces ofsub band images, the J being an integer equal to two or more; atransform device which transforms a luminance value of each pixel ofsaid sub band images to a brightness impression value based on apredetermined relation between luminance and brightness impression; anda synthesis device which performs wavelet synthesis of K pieces of subband images which have been subjected to the transformation of theluminance values to the brightness impression values by said transformdevice, to generate a brightness image, the K being an integer equal totwo or more; K≦J.
 2. An image transform apparatus comprising: aresolution device which performs wavelet resolution of a luminance imageN times to generate (3N+1) pieces of sub band images, the N being anatural number; a transform device which transforms a luminance value ofeach pixel of said sub band images to a brightness impression valuebased on a predetermined relation between luminance and brightnessimpression; and a synthesis device which performs wavelet synthesis of(3N+1) pieces of sub band images N times to generate a brightness image,the (3N+1) pieces of sub band images having been subjected to thetransformation of the luminance values to the brightness impressionvalues by said transform device.
 3. The image transform apparatusaccording to claim 2, wherein: said resolution device performs thewavelet resolution, using an orthogonal wavelet; and said synthesisdevice performs the wavelet synthesis, using the orthogonal wavelet. 4.The image transform apparatus according to claim 3, wherein theorthogonal wavelet is an approximately symmetrical function.
 5. Theimage transform apparatus according to claim 3, comprising: a secondresolution device which performs wavelet resolution of the brightnessimage generated by said synthesis device, to generate J′ pieces of subband images, the J′ being an integer equal to two or more; a secondtransform device which transforms, based on said relation, thebrightness impression value of each of the pixels of said sub bandimages generated by said second resolution device to the luminancevalue; and a second synthesis device which performs wavelet synthesis ofK′ pieces of sub band images which have been subjected to thetransformation of the brightness impression values to the luminancevalues by said second transform device, to generate a luminance image,the K′ being an integer equal to two or more; K′≦J′.
 6. The imagetransform apparatus according to claim 3, comprising: a secondresolution device which performs wavelet resolution of the brightnessimage generated by said synthesis device N′ times, to generate (3N′+1)pieces of sub band images, the N′ being a natural number; a secondtransform device which transforms, based on said relation, thebrightness impression value of each of the pixels of said sub bandimages generated by said second resolution device to the luminancevalue; and a second synthesis device which performs wavelet synthesis of(3N′+1) pieces of sub band images to generate a luminance image, the(3N′+1) pieces of sub band images being having been subjected to thetransformation of the brightness impression values to the luminancevalues by said second transform device.
 7. An image transform programfor causing a computer to execute: a resolution step of performingwavelet resolution of a luminance image to generate J pieces of sub bandimages, the J being an integer equal to two or more; a transform step oftransforming a luminance value of each pixel of said sub band images toa brightness impression value based on a predetermined relation betweenluminance and brightness impression; and a synthesis step of performingwavelet synthesis of K pieces of sub band images which have beensubjected to the transformation of the luminance values to thebrightness impression values by said transform step to generate abrightness image, the K being an integer equal to two or more; K≦J. 8.An image transform program for causing a computer to execute: aresolution step of performing wavelet resolution of a luminance image Ntimes to generate (3N+1) pieces of sub band images, the N being anatural number; a transform step of transforming a luminance value ofeach pixel of said sub band images to a brightness impression valuebased on a predetermined relation between luminance and brightnessimpression; and a synthesis step of performing wavelet synthesis of(3N+1) pieces of sub band images N times to generate a brightness image,the (3N+1) pieces of sub band images having been subjected to thetransformation of the luminance values to the brightness impressionvalues by said transform step.